Constraining near-term projections of the South Asian high and Afro-Asian summer monsoon rainfall.

Reliable projections of Afro-Asian summer monsoon precipitation (AfroASMP) are critical for sustaining regional development and managing climate risks for billions of people. The complex historical variability of AfroASMP highlights the central role of dynamical circulations, alongside the favorable thermodynamic conditions associated with the centurial global warming. Here, we identify that up to 90% of the variance in AfroASMP over past six decades is dynamically dominated by the South Asian High (SAH), an extensive upper-tropospheric mode spanning West Africa to the western Pacific. Using a multi-model weighting scheme, the constrained near-term SAH projects a larger amplitude than raw simulations across three emission scenarios (SSP1-2.6, SSP2-4.5, SSP5-8.5). SAH-based constraints indicate a reduction in AfroASMP of 10-50 mm, with the largest decrease in abnormally wet conditions over Africa (62%) under SSP5-8.5, and increases in dry conditions over Africa and North China (>10%). The framework offers physically grounded evidence for climate adaptation and policy decisions.